I'm planning on making an amendment to her lease with her "guest" on it stating liability and an increase of $30 per month to cover the utilities I cover for another adult.

They are a two adults, two children family.  

I also make the tenant identify who will be occupying the property and the lease states that only the identified folks can stay on the property (all adults get screened, even if they aren't signing the lease).  

You have a contract with two adults.  

Some states require landlords to look at the total income of the tenants, like CA, but others allow landlords to require that each adult can pay the entire rent on their own (3X the rent, etc.).So, it just depends.  

I have 5 adults (and 2 dogs) wanting to rent one of our brand new houses.

The secret life of bees: Children put things in sinks and toilets without adults noticing.

It is their problem to figure out like adults. 

Adult kids with limited funds vacationing with Grandma and Granoa who are footing the bills.  

Four adults should be able to easily afford to pay the rent, as agreed.